For a single man, when is one considered impure? Often following a prolonged erection, a mild and transparent liquid may come out by itself or when one's penis is pressed. If this is the case, is a man physically impure to pray and needs to perform ghusl? Finally, when one has wet dreams, the body's instinct often awakens you before semen disposal. If one wakes up and feels he shall ejaculate and forces himself not to succed in keeping the semen inside, does this avoid janaba and if it does is it healthy or should it be shunned?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Apparently as stated in your question, the fluid that flows from your erecting organ is al-Mazi (a transparent pre-seminal fluid). Such a case requires washing your organ and the spoiled spots of your clothes or body, and there is not need for al-Ghusl (ceremonial bath).
In the Prophetic Hadith recorded by al-Bukhari and Muslim, Ahmad and others that ' Ali (RAA) said: I used to find al-Mazi (on my clothes and body). So, I asked Allah's Messenger (SAW) and he said: "al-Mazi requires wudu and al-Mani (semen) requires al-Ghusl."'
Emission of al-Mani due to dream even after waking up requires al-Ghusl as long as al-Mani gushs out with delight. The Prophet (Sallallahu Alaihi wa Sallam) said: "If you eject al-Mani you have to take al-Ghusl; and if do not eject you do not have to do al-Ghusl" [ Ahmad and others]
Allah says: {….. If you are in a state of Janâba (i.e. had a sexual discharge), purify yourself (bathe your whole body)……. } [5:6]
As for some harm due to ejaculation, we do not know if there is or not, but we do not think that there is some harm since emission of al-Mani while in a dream is a natural thing, i.e. not masturbation.
Allah knows best.